Tornado: Urlencoded body parsing omits max_num_fields, so one request can stall the event loop
Summary
Tornado parses application/x-www-form-urlencoded bodies with urllib.parse.parse_qs and does not pass max_num_fields. A body made almost entirely of separators produces tens of millions of fields, and the parse happens on the event loop before the handler runs, so a single request stalls the whole server.
Where it is
tornado/escape.py, at HEAD e530031405e2154654dedc4c84d5656b557ea310:
result = urllib.parse.parse_qs(
qs, keep_blank_values, strict_parsing, encoding="latin1", errors="strict"
)max_num_fields is the parameter CPython added for exactly this, and it is absent.
The path to it is entirely server-side and pre-dispatch. RequestHandler._execute parses the body at tornado/web.py:1821, which reaches HTTPServerRequest._parse_body at tornado/httputil.py:636, and the urlencoded branch of parse_body_arguments calls parse_qs_bytes at tornado/httputil.py:1030.
The size that reaches it is bounded only by the body cap, which defaults to the stream's max_buffer_size of 104857600 at tornado/iostream.py:239, applied as the request body default at tornado/http1connection.py:136-140. A 100 MB body of separators is around fifty million fields.
Impact
Denial of service against the whole process, not one request. Tornado is single-threaded and the parse is synchronous on the event loop, so every other connection waits. No authentication is needed if any route accepts a form post, which is the normal case.
Suggested fix
Pass a bound:
result = urllib.parse.parse_qs(
qs, keep_blank_values, strict_parsing, encoding="latin1", errors="strict",
max_num_fields=max_num_fields,
)with a conservative default and a way for applications to raise it. CPython raises ValueError when the limit is exceeded, which maps cleanly onto a 400.
Lowering the default body cap for urlencoded specifically would help too, since 100 MB of form fields is not a shape any real client sends.
Why I do not think this is a duplicate
The published tornado advisories cover out-of-bounds access in the C extension, unbounded accumulation of decompressed chunks in AsyncHTTPClient, the Authorization header surviving cross-origin redirects, credential leakage on curl handle reuse, and cookie attribute validation. The decompression one is the nearest in spirit and is on the client side; this is the server parsing a request body. The call is unchanged at HEAD.
Czego dotyczy
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| Platforma | Paczka | Podatne wersje | Poprawka w wersji |
|---|---|---|---|
| Python (PyPI) | tornado | < 6.5.8 | 6.5.8 |
Odnośniki
- https://github.com/tornadoweb/tornado/security/advisories/GHSA-mpf4-983q-p7j4
- https://nvd.nist.gov/vuln/detail/CVE-2026-82397
- https://github.com/tornadoweb/tornado/pull/3704
- https://github.com/tornadoweb/tornado/commit/8d6363ed7b69d5f0da806efe34d256627a2191de
- https://github.com/tornadoweb/tornado
- https://github.com/tornadoweb/tornado/releases/tag/v6.5.8
- https://pypi.org/project/tornado
- https://github.com/advisories/GHSA-mpf4-983q-p7j4
- Wektor CVSS
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H- Słabość (CWE)
CWE-1284, CWE-400- Platforma
- Python (PyPI)
- Paczka
tornado- Ujawniono
- 02.09.2026
- Aktualizacja
- 11.10.2026 15:18
Źródła i licencje
OSV / GitHub Advisory Database - PYSEC-2026-3928 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-mpf4-983q-p7j4 - CC BY 4.0
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